The present invention generally relates to semiconductor devices, and more particularly to semiconductor devices having semiconductor light receiving elements which are suited for use in a coherent optical communication system.
Presently, there is a need for increasing the speed of communication systems, and coherent optical communication systems provide effective means for realizing high-speed communication.
The use of a heterodyne detection system in coherent optical communication systems has been proposed. The signal obtained is such system has a frequency which is the difference or the sum of the frequency of an actual optical signal and the frequency of an oscillation light from a local oscillator made up of a laser diode. For this reason, the signal-to-noise (S/N) ratio is improved and it is possible to improve the minimum signal reception level of an optical signal receiver.
A dual balanced optical receiver which is made up of two pin photodiodes is used as the optical signal receiver. Uniform optical and electrical characteristics are demanded of the two pin photodiodes of the optical signal receiver.
FIG. 1 is a diagram for explaining a dual balanced optical receiver made up of two pin photodiodes. The dual balanced optical receiver generally comprises pin photodiodes D1 and D2, a positive power source pad P1, a negative power source pad P2, a signal output pad P3, an optical waveguide LG1 for introducing an optical signal, an optical waveguide LG2 for introducing an oscillation light from a local oscillator, and an optical coupler OC which is made up of an optical fiber, a semiconductor or a dielectric material.
In this dual balanced optical receiver, the optical signal input from the optical waveguide LGI and the oscillation light input from the optical waveguide LG2 are mixed in the optical coupler OC and directed to the pin photodiodes D1 and D2. An electrical signal having a frequency which is the difference or the sum of the frequency of the optical signal and the frequency of the oscillation light is obtaine from the signal output pad P3. Of course, the selected frequency of the electrical signal is an intermediate frequency differs from the frequency of the optical carrier of the optical signal. In addition, the electrical signal obtained from the signal output pad P3 is amplified in an intermediate frequency amplifier and is then subjected to appropriate detection depending on modulation system so as to obtain a base band signal. For example, the appropriate detection procedure may involve envelope detection, delay detection, synchronous detection for the like.
When making the dual balanced optical receiver shown in FIG. 1, two pin photodiodes having approximately the same characteristics are selected and used as the pin photodiodes D1 and D2, and there are only a few examples of monolithic integrated circuits which have two pin photodiodes provided on the same semiconductor substrate. In order to further increase the communication speed of the coherent optical communication device, it is important that the characteristics of the two pin photodiodes be uniform and that the stray capacitances of the pin photodiodes and of the electrical interconnections be as small as possible. For this reason, it is considered essential that the semiconductor light receiving elements be integrated on the same semiconductor substrate in the form of a monolithic integrated circuit.
When a plurality of pin photodiodes are formed on the same semiconductor substrate, it should be easier to make the characteristics of the pin photodiodes uniform compared to a case where independent pin photodiodes are provided. However, depending on the layout of the elements of the monolithic integrated circuit, it has been impossible to make a dual balanced optical receiver having satisfactory optical and electrical balance characteristics.